1. Field of the Invention
This invention relates to a transparent electrically conductive film whose transmittance is increased, and to an optical element in which said transparent electrically conductive film is caused to generate heat and thereby eliminate fogging occurring on the optical element.
2. Related Background Art
Cameras such as television cameras, video cameras and photographic cameras are sometimes used in an environment in which temperature and humidity change suddenly.
For example, there is a case where photographing is carried out outdoors in the rainy season in which the rain on the previous day ceases and temperature rises due to the fine weather from the morning. Generally, if the temperature of the open air changes suddenly, the difference between the temperature in the camera and the temperature of the open air becomes great and fogging sometimes occurs on the lens surface of the photo-taking lens to make photographing impossible.
Therefore, for example, in the case of a television camera or the like, heating means has heretofore been provided in the camera so that when the environmental conditions of the open air change suddenly, the temperature in the camera is increased by the heating means to thereby prevent fogging from occurring on the lens surface.
However, it is very difficult to uniformly increase the temperature of each lens surface in the lens barrel and thereby prevent fogging from occurring on the entire lens surface, and it has often been the case that fogging inevitably occurs partly.
Also, even in the case of an optical apparatus used indoors, fogging may sometimes occur on the imaging lens thereof to reduce the performance thereof. In Japanese Utility Model Publication No. 62-41383, it is proposed to provide an electrically conductive film on the objective lens of a pick-up device for digital disk, supply an electric current to this electrically conductive film through an electrode and thereby warm the objective lens. This is considered to be an application of the technique of preventing condensation from occurring on the window glass of an automobile.
On the other hand, it has become popular to use a transparent electrically conductive film of indium oxide (In.sub.2 O.sub.3) or tin oxide (SnO.sub.2) as the transparent electrode of a liquid crystal panel.
This transparent electrically conductive film, if it is a single piece, is not high enough in transmittance.
As a method of decreasing the reflectance of a panel formed with a transparent electrically conductive film, there is known a film construction using a transparent electrically conductive film instead of a high refractive index layer, as a constituent film of a multilayer reflection preventing film formed on the surface of a substrate.
For example, with .lambda..sub.0 as the central wavelength of the reflection preventing area, there is made a film construction comprising an integer layer (a layer having an optical film thickness integer times as great as 1/4 of .lambda..sub.0).
(1) Substrate glass - In.sub.2 O.sub.3 (.lambda..sub.0 /2)-MgF.sub.2 (.lambda..sub.0 /4) PA1 (2) Substrate glass - Al.sub.2 O.sub.3 (.lambda..sub.0 /4)-In.sub.2 O.sub.3 (.lambda..sub.0 /2)-MgF.sub.2 (.lambda..sub.0 /4)
where parentheses represent the optical film thickness obtained by multiplying the physical film thickness by the refractive index.
However, with these constructions, the reflectance of the transparent electrically conductive film could be decreased, but the light absorption of the film itself could not be decreased. In the material forming the transparent electrically conductive film, electrons are ready to be excited from the value electron zone to the electrically conductive zone. Light energy is absorbed during this excitation and therefore, the transparent electrically conductive film is readier to absorb light than an ordinary optical film such as MgF.sub.2, ZrO.sub.2 or Al.sub.2 O.sub.3. In the reflection preventing film, this transparent electrically conductive film must be provided to a thickness corresponding to .lambda..sub.0 /2 for the central wavelength .lambda..sub.0. Where .lambda..sub.0 =500, a film of a refractive index 1.9 need be as thick as 132 nm, and the decrease in transmittance by light absorption is great to a degree which cannot be neglected, and the use of such film, for example, as an antifogging film in an optical element whose transmittance is necessary up to approximately 100% leads to a great loss.
SnO.sub.2 film (resistivity 2.times.10.sup.-3 .OMEGA..multidot.cm) which is a transparent electrically conductive film being presently used has a transmittance of only 85% for a film thickness of 125 nm, and likewise, In.sub.2 O.sub.3 film (resistivity 2.times.10.sup.-4 .OMEGA./cm) used as a transparent electrically conductive film provided a transmittance of only 90% for a film thickness of 125 nm.